graphs_controller.rb

# app/controllers/graphs_controller.rb
def show
  ...
  @presenter = GraphPresenters::LineGraphPresenter.new graph
end

line_graph_presenter.rb

# app/presenters/graph_presenters/line_graph_presenter.rb
module GraphPresenters
  class LineGraphPresenter
    public

    def initialize( graph )
      @graph = graph

      @options = {
        :number_of_vertical_lines = 11,
        :range_top_value = @graph.max_value
      }
      @options[:range_step_size] = @options[:range_top_value] / @options[:number_of_vertical_lines]
    end

    def configure( settings = {} )
      settings.assert_valid_keys( :width, :height )
      @options.merge! settings
      @options.merge! {
        x_start: 113, # Leave space for the vertical labels
        y_start: 10,  # Leave space for the year-labels
        radius: 5,    # The radius of each data point
      }
      @options.merge! {
        x_end: @options[:width] - @options[:radius],
        y_end: @options[:height] - @options[:radius]
      }
    end
    
    def vertical_line(n)
    {
      x1: vertical_line_position_x(n),
      y1: @options[:y_start],
      x2: vertical_line_position_x(n),
      y2: @options[:y_end]
    }
    
    def horizontal_line(n)
    {
      x1: @options[:x_start],
      y1: horizontal_line_position_y(n),
      x2: @options[:x_end],
      y2: horizontal_line_position_y(n)
    }
    end
    
    def circle(n)
      value = @graph.datapoint n
      line = vertical_line n
    
      {
        cx: line[:x1],
        cy: y_position value,
        r: 5
      }
    end
    
    def surface
      point = circle 0
      first = point
      path = "M#{point[:cx]},#{@options[:y_end]}"
    
      @graph.number_of_datapoints.times do |n|
        point = circle n
        path += "L#{point[:cx]},#{point[:cy]}"
      end
    
      path << "L#{point[:cx]},#{@options[:y_end}"
      path << "M#{first[:cx]},#{@options[:y_end}"
    
      { d: path }
    end
    
    private
    def distance_between_vertical_lines
      pixels_available = (@options[:x_end] - @options[:x_start])
      pixels_available / @graph.number_of_datapoints
    end
    
    def distance_between_horizontal_lines
      pixels_available = @options[:y_end] - @options[:y_start]
      pixels_available / @options[:number_of_vertical_lines]
    end
    
    # Vertical lines are aligned with each year on the x-axis
    def vertical_line_position_x(n)
      @options[:x_start] + (distance_between_vertical_lines * n)
    end
    
    # Horizontal lines provide visual guidance for the value of data points
    def horizontal_line_position_y(n)
      @options[:y_start] + (distance_between_horizontal_lines * n)
    end
    
    def y_position(value)
      # Assume the y-axis always starts from 0
      pixels_per_value = (@options[:end] - @options[:start]) / @options[:range_top_value]
      @options[:y_end] - (value * pixels_per_value)
    end
  end
end

show.html.haml

# app/views/graphs/show.html.haml
%svg{xmlns: 'http://www.w3.org/2000/svg','xmlns:xlink' => 'http://www.w3.org/1999/xlink', version: '1.1', class: 'graph'}
  %g.grid#xGrid
    - @presenter.number_of_datapoints.times do |n|
      %line{ @presenter.vertical_line n }

  %g.grid#yGrid
    - @presenter.number_of_vertical_lines.times do |n|
      %line{ @presenter.horizontal_line n }

  %g.surfaces
    %path{ @presenter.surface }

  %g.points
    - @graph.number_of_datapoints.times do |n|
        %circle{ @presenter.circle(n) }